Add soy sauce, garlic, brown sugar, pineapple juice and apple cider vinegar to a small pot. Bring to a boil then simmer over low heat for 2 minutes. Remove from heat and pour the marinade into a large shallow dish (to marinate steaks later). Set aside until it’s cooled, or to speed up the process, cover and place it in the fridge until it’s cool.
Add steak into the dish with marinade. Cover and place in fridge for 1 hour. Flip sides and cover for another 1 hour in the fridge. I prefer to marinate over night.
20 minutes before starting the grill, take dish with steaks out (still covered) to cool to room temperature. Heat grill.
Heat grill to high and grill steaks for 5 minutes. Flip and continue to grill for 4 minutes (for medium-rare to medium). Use an internal thermometer (135F for medium-rare and 140F for medium). Transfer steaks to a platter and tent with foil to let rest for 5 minutes. Grill some pineapple slices along with the steaks for garnish.